Welding wear-resistant steels like Hardox requires far more care than welding standard structural steel, because incorrect heat input can cause the material to lose the high hardness it was engineered for, right in the weld zone. In this article, we cover how preheating and heat input control should be handled when welding Hardox.
Why Does Welding Hardox Require Special Attention?
Hardox's high hardness comes from a special heat treatment process (quenching/tempering). The high temperature generated during welding can disrupt the microstructure achieved by this heat treatment; excessive heat input can cause softening in the heat-affected zone (HAZ) around the weld bead, or conversely, lead to cracking from excessive hardening.
Preheating Requirements and Temperature Values by Thickness
Preheating in Hardox welding is applied to reduce the risk of cold cracking; generally, preheating is not required for thicknesses below 8 mm, while as thickness increases (particularly above 15-20 mm) preheating in the range of 75-100°C is recommended. Exact values should be verified against the welding guidelines published by the material manufacturer, since different Hardox grades (400, 450, 500) may have different requirements.
Heat Input Calculation and Control
Heat input is calculated based on welding current, voltage, and welding speed (in kJ/mm) and must not exceed the maximum value specified by the manufacturer. Excessive heat input causes grain growth and hardness loss in the HAZ, while too little heat input can cause insufficient penetration and increase the risk of cold cracking. For this reason, welding parameters must be tightly controlled within the manufacturer's recommended range.
Electrode and Welding Method Selection
Low-hydrogen electrodes, or low-hydrogen wire for MAG welding, are generally preferred for Hardox welding, as this reduces the risk of cold cracking. Filler metal strength is often not fully matched to the base material; instead, a filler metal that does not exceed the base material's hardness but provides adequate strength is chosen, preventing excessive stress buildup in the weld zone.
Interpass Temperature Control
In multi-pass welds, the interpass temperature must not exceed the specified maximum (generally in the 150-200°C range, depending on material thickness). An increase in interpass temperature raises cumulative heat input, increasing hardness loss in the HAZ. Temperature should be measured regularly using a contactless infrared thermometer.
Post-Weld Cooling Management
Allowing the part to cool in a controlled manner after welding prevents stress buildup from sudden temperature changes. For some critical applications, using a controlled cooling blanket after welding is recommended.
Common Mistakes
One of the most common mistakes is applying preheating only at the start of the weld and not monitoring temperature as welding progresses. Another mistake is, in joints connecting Hardox parts of different thicknesses, basing preheat temperature on an average value rather than on the thicker part — this can cause excessive heat input on the thinner side.
DMK Makina's Approach to Hardox Welding
Based in Lüleburgaz and serving customers across the Thrace and Marmara regions, DMK Makina applies preheating and heat input control in line with manufacturer welding procedures for every thickness and grade in its Hardox welded fabrication services, preserving the material's wear resistance in the weld zone as well.
Conclusion
Correct preheating and heat input control in Hardox welding is the key to preserving the hardness and wear resistance the material is engineered for, within the weld zone. Strict adherence to manufacturer recommendations and regular temperature monitoring prevents cracking risk and ensures durable, reliable welded joints.
